Question

# A small droplet of oil of mass 1.44 * 10−15 kg having a charge of -1.54...

A small droplet of oil of mass 1.44 * 10−15 kg having a charge of -1.54 * 10−18 C accelerates straight down inside a vacuum chamber. If the electric field pointing up is 2500 N/C, find its acceleration. (Consider both gravitational and electric forces acting on the droplet of oil and pay attention to the directions of the forces with down being negative and up positive.)

 a. -10.28*10-15 b. -17.98*10-15 c. None of the given answers d. -7.14 e. -12.49 f. 0

Using Force balance on electron traveling in downward direction invacuum chamber:

Fy_net = -Fe - W

W = gravitational force on droplet in downward direction = m*g

Fe = Force due to electric field on negative charge in downward direction = |q|*E (Since force on negative charge is in opposite direction of electric field)

So,

m*a = -|q|*E - m*g

a = -|q|*E/m - g

Using given values:

a = -1.54*10^-18*2500/(1.44*10^-15) - 9.81

a = -12.48 m/s^2

Correct option is E.

Let me know if you've any query.